BBC News reports: Juergen Todenhoefer spent six days in the IS city of Mosul in Iraq, travelling there via Raqqa, in Syria.
Mr Todenhoefer said he found IS followers highly motivated and supportive of the group’s brutality.
He said the spread of fighters meant they were hard targets for air strikes.
A former German politician, Juergen Todenhoefer is the only outsider to have travelled deep into IS territory and back. And, considering that several Westerners have recently been beheaded, he did so at terrifying risk.